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Cloudflare Dynamic DNS Update Script for Asuswrt-Merlin (on supported ASUS routers)

The Asuswrt-Merlin custom firmware adds support for custom dynamic DNS providers to various ASUS routers. This is great for Cloudflare users because, although Cloudflare is not one of the built-in providers, we can add support for it. This guide and accompanying script do exactly that. Confirmed works on the following model routers:

  • GT-AX11000,
  • RT-AC3200,
  • RT-AC66U,
  • RT-AC68U,
  • RT-AX86U,
  • RT-AC87U, and
  • RT-AX88U.

Features include:

  • Support for querying your Cloudflare DNS zone to determine record IDs
  • Handling of Merlin firmware success/failure callbacks
  • Logging of JSON responses for later inspection, and
  • Configurable rate-limiting to comply with Cloudflare API TOS

Contributors

  • @epicylon - Verified works on RT-AC66U
  • @bengalih - Several contributions toward support for API Tokens, proxying, and other enhancements.
  • @gumanov - Verified works on RT-AX88U
  • @clayauld - Verified works on RT-AC87U
  • @ilasoft - Verified works on RT-AX86U
  • @sujitph - Verified works on GT-AX11000
  • @Rapsssito - Verified works on RT-AC3200

How to Configure

Prerequisites

You should have your Merlin-enabled ASUS router configured for your network with Internet access. Since you've found this guide, it's also assumed you have a Cloudflare account managing your own domain, and you've already created a subdomain you will use for dynamic DNS.

Configuration Overview

Configuration of Cloudflare DDNS involves changes through the router web portal as well as changes made through the router shell.

  1. Enable shell access and JFFS partition
  2. Install Cloudflare DDNS script
  3. Enable custom DDNS
  4. Verification
  5. Clean up

Directions for disabling dynamic DNS and removal of the script and related files are at bottom.

Enable Shell Access and JFFS Partition

In the router portal, under Administration -> System,

  • Persistent JFFS2 partition -> Enable JFFS custom scripts and configs: Yes
  • Service -> Enable SSH: LAN only

Save the configuration. Ensure you are able to SSH into your router using your router portal credentials (or via public key crypto, depending on configuration) before continuing.

Note: If SSH will be left enabled after installation, disallow password login, enable brute force protection, and use public keys for login to enhance security.

Install Cloudflare DDNS Script
  1. Log into your router via SSH, and navigate to /jffs/scripts.
  2. Copy the cloudflare_ddns and .cloudflare.example files to that directory.
  3. Rename .cloudflare.example to .cloudflare.
  4. Edit .cloudflare with your Cloudflare API token and zone ID from your Cloudflare portal. The script also supports the legacy "API Key plus account e-mail" method of authentication, but this method is less secure and appears likely to be eliminated in future.
  5. Run chmod 700 cloudflare_ddns.
  6. Run chmod 600 .cloudflare.
  7. Run ./cloudflare_ddns list.
  8. Step 7 should have resulted in the creation of a log file named cloudflare_ddns.log. Open the log file and review the JSON response object, which should be a listing of your Cloudflare DNS records for the zone ID specified in Step 4.

Note: If there is an error in the log file or no log file is present, ensure permissions are correct and that the text of the script is copied accurately. Double-check your Cloudflare credentials. For large Cloudflare configurations, it may be necessary to increase the log file size limit in the script. If the error is from Cloudflare, you can review the text of the error in the JSON response and look for any error code online.

  1. Edit .cloudflare with the DNS record information (i.e. ID, name and type) obtained from Step 8. Ensure your text matches exactly.
  2. Run ./cloudflare_ddns 1.1.1.1.
  3. Review the log file for the result of the last execution. If you see a successful response, verify against the Cloudflare portal. Otherwise, review the errors and correct as necessary.

Note: You may get a throttled response if you have queried too quickly after Step 7. The script rate-limits to one query every 5 seconds. This is configurable in the cloudflare_ddns script or you can simply wait.

  1. Ensure rate-limiting is working as expected by re-issuing the command in Step 10 a couple of times in quick succession and verifying that the log file shows frequent invocations are throttled.
  2. If Steps 11 and 12 were successful, run ln -s cloudflare_ddns ddns-start. This creates a symbolic link with the name expected by the router firmware.
Enable Custom DDNS

In the router portal, under WAN -> DDNS,

  • Enable the DDNS client: Yes
  • Server: Custom
  • Host name: the DNS host name you're using in Cloudflare
  • HTTPS/SSL Certificate: None

Save the configuration.

Verification

If all is configured correctly, you should see:

  1. The DDNS Registration Result field indicating success.
  2. In the router portal, under System Log, you should see entries as below.
Nov 5 6:57 start_ddns: update CUSTOM , wan_unit 0
Nov 5 6:57 custom_script: Running /jffs/scripts/ddns-start (args: x.x.x.x ) - max timeout = 120s
Nov 5 6:57 ddns: Completed custom ddns update
  1. A new log file should have been created, and it should contain a successful log entry.
  2. The Cloudflare portal should reflect the updated public IP address of your router.

Note: If any errors occur, review the router log file and the script log file for an indication of the error or manually re-run ./cloudflare_ddns list and ./cloudflare_ddns 1.1.1.1 to identify and troubleshoot.

Clean Up

Once everything is configured and working properly, you may delete the cloudflare_ddns.log file from the /jffs/scripts/ directory on the router. If SSH access is no longer needed, disable SSH on the router portal for security (especially if password authentication was used).

Script Removal

To remove the script, the process is essentially reversed.

  1. In the router portal, disable DDNS client and save. It may be worthwhile to restart your router to ensure any in-memory settings are cleared.
  2. Log into the router via SSH and delete (in order): a) ddns-start, b) cloudflare_ddns, c) .cloudflare, d) cloudflare_ddns.log, and e) ddns-start.log.